I personally believe that Twilight, if she were so inclined, could potentially develop an attraction to any of the girls. I think it's in her nature to be experimental, even with romance, and she'd probably shuffle back and forth on it, think about the girls a lot, make checklists and all that typical Twilight stuff. Especially with the dynamic she has as a pseudo leader...
That said, I think TwiPie is my personal favorite for all the reasons stated above, and it also neatly rounds out my trifecta of ships, AppleDash, RariShy, and then TwiPie. THE HOLY TRINITY MUST REMAIN INTACT